The present invention relates generally to an illumination system in an electrophotographic printing machine and more particularly, to a full-frame flash illumination system contained within a light housing.
As demands for faster copying and duplicating have increased, conventional machines which scan documents in incremental fashion to provide a flowing image on a xerographic drum have proved inadequate. New high speed techniques have evolved which utilize flash exposure of an entire document and the arrangement of a moving photoconductor in a flat condition at the instant of exposure. Full efficiency of such prior art systems has been realized by enclosing the illumination system within a light housing. The interior walls of such a housing may be diffusely or specularly reflective to produce multiple reflections of light emanating from the illumination source contained within the housing. A uniform level of illumination is produced at the object plane; generally a transparent platen upon which the document to be reproduced is positioned. U.S. Pat. No. 4,250,538 is representative of such systems.
In these prior art systems, the imaging lens is positioned within the floor of the light housing and, in a multi-magnification mode, is moved laterally and vertically to positions required by magnification changes. One problem with these prior art imaging systems is that an image of the lens face is reflected from the bottom surface of the transparent platen and is projected back through the lens onto the photoreceptor surface. This negative disc image termed a "black hole" is subsequently developed as part of the document image and, upon transfer to a recording sheet, appears as a relatively dark spot on the output copy. Various expedients have been tried to reduce this problem. For example, as disclosed in U.S. Pat. No. 4,250,538, the lens barrel is painted white to reduce the size of the reflected lens image. In copending U.S. application Ser. No. 608,300 the unwanted lens image is located at the photoconductor and discharged to a lower level by an appropriately positioned illuminator.
It is therefore an object of the present invention to reduce or eliminate the "black hole" problem in full-frame flash-type systems. It is a further object to improve the invention disclosed in the aforementioned U.S. Application. This object is accomplished, according to the invention by providing a flash illumination and optical imaging system for reproducing documents on an object plane onto a photoreceptor belt member comprising, in combination:
a transparent object plane for supporting original documents to be reproduced;
an enclosed document illumination housing positioned beneath said object plane; PA1 an illumination means positioned interior of said housing and adapted to be periodically energized to provide a generally uniform level of illumination at said object plane; PA1 a projection lens mounted in the bottom surface of said housing opposite said object plane, said lens adapted to project an image of said document onto said photosensitive belt, said projected image containing a superimposed image of the projection lens face as reflected from said object plane; and PA1 means for directing a portion of the illumination produced within said housing onto the area of said belt conforming to the area of said negative lens image so as to reduce the charge on said lens image area.